Stents are tubular molds which are made of biocompatible materials and are contracted on their introduction and then dilate for their stable insertion into the desired duct or lumen. Some types of stents dilate mechanically, and for their insertion they require the use of an expandable element arranged along the introducing instrument and acting inside the stent.
As the instrument for the introduction of mechanically dilating stents, a catheter having an inflatable balloon at its distal end of the type already used may also be used, e.g., for dilating arterial ducts or other lumina in a live body. Therefore, for the introduction operation, the balloon, empty, is wrapped tightly around a corresponding zone of the catheter, and the stent, in its turn, is arranged tightly around the balloon in order to remain fixed there during the introduction into a lumen.
In general then, the catheter, at the balloon, has a radiopaque marker in order to be able to control the position of the catheter and, therefore, that of the balloon in the duct or lumen in question.
However the requirements of a catheter for the simple dilation of ducts or lumina most often are incompatible with those of a catheter for the positioning of a stent, which is why specific catheters would be needed for one or the other operation.
A dilating catheter, for example, for angioplasty procedures, or the like, must have a small diameter, a low coefficient of surface friction, and a highly resistant balloon. On the other hand, a catheter for the positioning of stents must have a remarkable friction, at least at the balloon, be made of a material having a surface that is adapted to the interior of the closed, i.e., contracted stent, and have a diameter of the balloon, when closed, i.e., wrapped around the catheter, that is at least slightly greater than the internal diameter of the closed stent. All this is to hold the stent and to prevent its loss during the insertion in a duct or lumen.
Otherwise, a diameter in the zone of the catheter, including the closed balloon wrapped there, that is smaller than the internal diameter of the closed stent that is applied there may cause:
a so-called overlapping resulting from a corrugation and an overlapping of some parts of the stent with possible deformation of its structure, if the stent is contracted too tightly in order to adhere to such a zone; or PA1 an improper fixing of the stent with the possibility of losing it during the introduction if the stent, although suitably and correctly closed, does not adhere to the too-small outer surface of the catheter plus closed balloon.